In a reciprocating four-stroke piston engine, each piston cylinder has an intake valve and an exhaust valve. The intake valve opens on a downstrike of the piston and an air-fuel mixture is drawn into the cylinder chamber. The mixture is compressed on the ensuing upstroke. Combustion is initiated at the top of that upstroke. The combustion forces the piston through a downstroke. The exhaust valve opens to expel gases on the final upstroke of the four-stroke cycle. The four-stroke cycle then begins again with the opening of the intake valve and a downstroke of the cylinder. A cam on a camshaft controls the opening and closing of each valve, respectively. In common engine design, there is a period during which the intake valve and exhaust valve are open simultaneously. This occurs during the end of the exhaust upstroke and the beginning of the intake downstroke when the intake valve opens prior to complete closing of the exhaust valve. This simultaneous open-valve position is called "valve overlap." In many instances it would be desirable to reduce or otherwise modify the duration or instant of occurrence of valve overlap. An important reason would be to increase engine efficiency by not exhausting some of the air-fuel mixture with gases during the exhaust upstroke or by not mixing gases which are combustion by-products with the air-fuel mixture.
It can be appreciated that it would be desirable to have a means for controlling valve overlap in a reciprocating piston engine. It can also be appreciated that it would be desirable to have a means for varying intake and exhaust valve opening and closing in a reciprocating piston engine.